The present invention is directed to plastic container closures for beverage, food, juice, pharmaceutical and like applications, and more particularly an improved process for providing closures with sealing liners having resistance to transmission of gases, water vapor and/or flavorants (flavor scalping).
Reference is made to concurrently filed application Ser. No. 08/997,871 filed Dec. 24, 1997, now U.S. Pat. No. 6,399,170, entitled xe2x80x9cPlastic Closure with Compression Molded Barrier Linerxe2x80x9d and assigned to the assignee hereof.
It has heretofore been proposed to provide a plastic closure for a container that comprises a plastic cap with an interior liner for sealing engagement with the sealing surface of the container. For example, U.S. Pat. No. 4,984,703 discloses a plastic closure that comprises a cap having a base with a peripheral skirt and threads for securing the cap to a container, and a sealing liner compression molded in situ to the interior of the cap base. The sealing liner comprises a blend of ethylene vinyl acetate (EVA) and a thermoplastic elastomeric material such as olefin or styrene-butadiene-styrene. U.S. Pat. No. 5,451,360 discloses a method and apparatus for compression molding the liner in situ within the caps.
Although the closures and methods of manufacture disclosed in the noted patents address problems theretofore extant in the art, further improvements remain desirable. For example, although soft olefin copolymers such as EVA are sufficiently resilient to provide good sealing against the sealing surface of a container when the closure is fastened to the container, these materials do not provide an acceptable barrier against transmission of gases such as oxygen and carbon dioxide that can deleteriously affect the product within the container. It has heretofore been proposed to employ a barrier material such as ethylene vinyl alcohol (EVOH) as a gas transmission barrier layer. However, materials of this character tend to be expensive and brittle, and are not well suited to function as a seal. It is therefore a general object of the present invention to provide a liner for a plastic closure that combines the functions of a seal for engagement with the container sealing surface and an improved barrier against gas transmission, flavor absorption (flavor scalping) and/or water vapor permeation. Another and more specific object of the present invention is to provide a liner of the described character that is of readily moldable and inexpensive composition. Yet another object of the invention is to provide a liner that satisfies the foregoing objectives and is of clear or translucent construction to permit reading through the liner of printing on the closure. A further object of the present invention is to provide a method of fabricating such a liner, and a plastic closure embodying such a liner.
A plastic closure in accordance with one aspect of the present invention comprises a plastic cap having a base with a peripheral skirt defining the interior of the cap and threads or other suitable means on the skirt for securing the closure to a container. A liner is secured to the interior of the base. The liner consists essentially of a multiplicity of alternating layers of a matrix polymer and a barrier material to resist transmission of gas through the liner parallel to the plane of the liner. The liner in the preferred embodiment of the invention is compression molded in situ within the cap, and includes at least nine alternating layers of matrix polymer and barrier materials, preferably at least thirty-three alternating layers, and most preferably one hundred twenty-nine alternating layers.
The xe2x80x9cmatrix polymerxe2x80x9d is a thermoplastic elastomer, a soft olefin polymer, or a combination thereof. A thermoplastic elastomer is a synthetic polymer having the processability of a thermoplastic material and the functional performance and properties of a conventional thermoset rubber. There are six generic classes of thermoplastic elastomer commercially available, including styrenic block, copolymers (SBC), polyolefin blends (TPO), elastomeric alloys, thermoplastic polyurethanes (TPU), thermoplastic copolyesters and thermoplastic polyamides. Thermoplastic elastomers are described beginning at page 64 in Modern Plastics Encyclopedia Handbook, published by McGraw-Hill, 1994, the disclosure of which is incorporated by reference. Examples of thermoplastic elastomers are styrene block copolymers as manufactured by Shell Chemical under the trademark KRATON. These synthetic polymers consist of three discrete blocks of the linear or A-B-A type: styrene. An elastomeric alloy is ethylene-propylene-diene terpolymer (EPDM). Another elastomeric alloy consists of compounds of EPDM/PP and butyl rubber/PP as manufactured by Advanced Elastomer Systems under the tradenames SANTOPRENE and TREFSIN and disclosed in U.S. Pat. Nos. 4,130,535, 4,311,628, 4,130,534 and 4,607,074. In general, thermoplastic elastomers are characterized by a Shore A hardness of 45 to 95 and a flexural modulus of 30,000 to 100,000 psi.
Soft olefin polymers are thermoplastic olefins, homopolymers and copolymers which are flexible, elastic with a Shore A hardness of less than about 100. Typical soft olefin polymers are: metallocene-made polyethylene, ethylene-propylene rubbers, ethylene copolymers and blends thereof, ethylene copolymers such as ethylene vinyl acetate, ethylene methyl acrylate copolymers and ionomers and combinations thereof. Examples of soft olefin polymers are alpha olefin substituted polyethylenes manufactured using single site catalyst technology (these materials are known in the art as metallocene-made polyethylenes); ethylene vinyl acetate (EVA) such as manufactured by DuPont under the trademark ELVAX; polypropylene made with single site catalyst technology known in the art as metallocene-made polypropylenes; syndiotactic polypropylenes as marketed by Fina Oil and Chemical; ethylene/propylene copolymers and styrene-ethylene interpolymers as marketed by Dow Chemical; and ionomers such as DuPont""s SURLYN product line.
The matrix polymer is typically compounded with anti-oxidants, lubricants and other stabilizing materials, as known in the art.
A xe2x80x9ccompatibilizerxe2x80x9d is a thermoplastic that ties two other thermoplastics together by a reactive (covalent or dipolexe2x80x94dipole) bond or a non-reactive (chain entanglement) means. Examples include maleic anhydride grafted polymers or ethylene vinyl acetate grafted polymers such as Quantum Chemical""s PLEXAR (trademark), Mitsui Petrochemical""s ADMER (trademark) and DuPont""s BYNEL (trademark) product lines, ethylene methyl acrylate, and ionomers.
A xe2x80x9cbarrier materialxe2x80x9d is a thermoplastic material that has a low gas and/or water vapor transmission rate and a high barrier to odorants and essential oils. The following materials have gas transmission rates lower than EVA, which is an industry standard liner material: EVOH (ethylene vinyl alcohol) such as Nippon Goshei""s SOARNOL (trademark) product line and Evalca""s EVAL (trademark) product line, nylons such as DuPont""s SELAR (trademark) PA, EMS""s G21 and Mitsubishi Gas"" MXD6 product lines, British Petroleum""s BAREX (trademark) acrylonitrile product line, blends of EVOH and amorphous nylon, blends of EVOH and an ionomer such as SURLYN (DuPont), and cyclic olefin copolymers such as marketed by Ticona. Other suitable barrier materials are blends as disclosed in U.S. Pat. Nos. 4,977,004 and 5,064,716, and nanocomposites of EVOH or nylon and clay as disclosed in U.S. Pat. Nos. 4,472,538 and 5,552,469, the disclosures of which are incorporated herein by reference.
It is currently preferred that the liner also include an additive for reducing the coefficient of friction between the liner and the sealing surface of the container. Friction reducing additives include metal stearates, microcrystalline waxes, polyethylene glycols, fatty acid esters and amides. These are known as xe2x80x9clubricantsxe2x80x9d in the art. The preferred lubricant is a low molecular weight fatty acid amide material that blooms to the exposed surface of the polymer material upon cooling from the melt state, thereby reducing the coefficient of friction between the liner and the container sealing surface. Examples are: primary amides with the general chemical structure Rxe2x80x94COxe2x80x94NH2, where R is an alkyl group; secondary amides with the general chemical structure Rxe2x80x94COxe2x80x94NHxe2x80x94Rxe2x80x2; where R, Rxe2x80x2 are alkyl groups; secondary bis-amides with the general chemical structure Rxe2x80x94COxe2x80x94NHxe2x80x94Axe2x80x94NHxe2x80x94COxe2x80x94R, where R, Rxe2x80x2 are alkyl groups and A is an alkylene group; and blends of the above materials such as in U.S. Pat. No. 5,306,542. The lubricant preferably comprises about 0.5% to 1.5% of the total liner composition by weight, most preferably about 0.5% by weight. The lubricant is preferably compounded into the matrix polymer material (along with any desired colorants) by the material manufacturer. The amount of lubricant and/or colorant is not included in the calculations of compositions in this application.
The barrier material and the matrix polymer in the liner are each in the amount in the range of about 2% to 50% by weight. The barrier material most preferably is provided in an amount in the range of about 6% to 35% by weight in the liner, the compatibilizer material preferably is in the range of about 6% to 20% by weight, the balance consisting of the matrix polymer.
In accordance with a second aspect of the present invention, a method of making a liner for a plastic closure comprises the steps of extruding a pellet that consists of a multiplicity of alternating layers of a matrix polymer and a barrier material that resists gas transmission, and compression molding the pellet to form a liner disk in which the alternating layers are oriented generally parallel to the plane of the disk. The layers in the pellet preferably are coextruded from inputs of barrier material, matrix polymer and compatibilizer. These materials may be separately extruded, or the compatibilizer may be mixed with the barrier material, the matrix polymer or both prior to extrusion. In the preferred implementation of the invention, the layers in the pellet are coextruded from a second input consisting of a blend of the matrix polymer and a compatibilizer material that promotes adhesion between the material layers. The step of compression molding the liner preferably is carried out by compression molding the liner in situ within a closure cap.
Thus, in accordance with a third aspect of the present invention, there is provided a sealing liner for a plastic closure that comprises a disk that consists essentially of a multiplicity of alternating layers of matrix polymer material and a barrier material that resists transmission of oxygen and carbon dioxide through the layers. The sealing liner preferably is compression molded in situ within a plastic closure from a compression mold charge or pellet in which the alternating layers are coextruded.